Neutral point clamped inverter circuits known in the art divide an input voltage using two capacitors connected in series, and output, for example, a three-phase AC (refer to, for example, Patent Literatures 1 to 3). Such an inverter circuit can be used to form a power conditioner that provides a three-phase AC to the utility grid during grid-connected operation, and has a single-phase three-wire output during isolated operation.
However, the two capacitors for dividing the input voltage can have an imbalance between their voltages (voltages across the two capacitors can differ from each other) depending on the use condition of the power conditioner including the neutral point clamped inverter circuit. When the power converter has such an imbalance between the capacitor voltages, the power converter cannot provide a normal output voltage, or can have an overcurrent or an overvoltage applied to the components of the inverter circuit, which may then damage the inverter circuit.